Apparatus for dispensing liquids and hot melts are known. For example, in U.S. Pat. Nos. 4,059,714 and 4,059,466, methods are disclosed for making and using hot melt foamed adhesives which offer significant improvements over conventional non-foamed adhesives. According to the techniques described in these patents, the gas is intimately mixed with a thermoplastic adhesive while the adhesive is in a molten state and then the mixture is pressurized to form a solution which, upon dispensing at a lower pressure, releases the gas and foamed adhesive results. Foam coating methods are also disclosed in U.S. Pat. No. 4,156,754, in the names of Walter H. Cobbs, Jr. and Robert G. Shong and application Ser. No. 842,265, filed Oct. 14, 1977, now U.S. Pat. No. 4,247,581, in the names of Walter H. Cobbs, Jr., Robert G. Shong and William R. Rehman. Foam coating involves coating high polymeric materials by first foaming liquid containing film-forming solids, then conveying the foam toward a surface to be coated and forming a film or solids on the surface.
A wide variety of liquid compositions, mostly of a polymeric nature, are employed in the adhesive and coating techniques disclosed in these patents and applications. Generally, the liquid or polymeric components of the compositions are employed in a heated liquid state. Upon subjecting such compositions to heat, especially for prolonged periods, they tend to either degrade, cure or are otherwise heat sensitive. Furthermore, when a liquid coating or polymer is employed with a foaming agent, such as methanol, high temperatures for foaming are required, for instance, in the range of about 200.degree.-235.degree. F. with polyester paints in order to get good foaming and atomization. In such cases, a higher temperature will tend to cure catalyzed polyester paint within a relatively short period of time, i.e., about 1-2 hours. Often paint line operations require as high as 10 hours of duration with circulation of hot paint to the gun or applicator free of electrical heaters. Therefore, further improvements need be made that permit dispensing of such foamable compositions at satisfactory temperatures and for longer periods of time. It is also necessary to provide adequate mixing and uniform foam-volume ratios to allow processing of liquid foaming systems.
Other problems are encountered with heat sensitive liquids. For instance, organosols and plastisols are widely used throughout industry in many diverse applications. However, when such compositions are used with liquid dispensers, they exhibit heat sensitivity and tend to fuse or thicken. As a result, the fused or thickened material either clogs up equipment, causes equipment breakdown, or makes it difficult to maintain an effective and smoothly running liquid dispensing system.
The above brief background exemplfies the need for further improvements in dispensing liquids, melt compositions and heat sensitive liquids with compositional uniformity during dispensing or prolonged or workable periods.